bird, v.² — Green’s Dictionary of Slang

bird v.²

(US) to talk nonsense; usu. in phr. not (just) birding.

1947		Berrey & Van den Bark Amer. Thes. Sl. (Supplement).
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 264: And you ain’t just a-birdin’, son. You ain’t a birdin’ a-tall.
1953		Felsen Street Rod 82: You’re not just a-birding, Dad [HDAS].
1965		W. Hoffman Yancy’s War 21: ‘They deserve recognition.’ ‘You ain’t birding.’ [HDAS].